The present invention relates to a press used for producing multilayer circuit boards, and more particularly, to a hot press which is suitable for forming a high-density and high-quality multilayer circuit board at low cost.
A multilayer circuit board is generally produced in the following manner. Namely, a predetermined number of printed circuit boards each printed with a predetermined circuit pattern and a predetermined number of prepregs formed by a cloth base material impregnated with a bonding resin are stacked in layers and, then, the materials of multilayer circuit board comprising the printed circuit boards and the prepregs thus stacked in layers are heated and pressed by hot plates. Accordingly, at the same time when the temperature is raised, the bonding resin softens to fluidify to reach its minimum viscosity, and thereafter, with the progress of chemical reaction, it becomes hard to be stable. As a result, the plurality of printed circuit boards are bonded to each other by the bonding resin of the prepregs to be one body, a multilayer circuit board is produced.
In order to enhance the density of the above-described multilayer circuit board, it is necessary to prevent the oxidation of junctions and to remove voids from between the layers of the multilayer circuit board. Accordingly, it has been proposed to carry out the hot press operation in a high vacuum.
In Japanese Patent Unexamined Publication No. 62-156931, for example, it is disclosed that the materials of the multilayer circuit board are covered with a film or a sheet and, after reducing the pressure inside the film or the sheet, the materials of the multilayer circuit board are heated and pressed by the hot plates.
Further, Japanese Patent Unexamined Publication Nos. 61-43543 and 61-43565 disclose a method which makes use of a high pressure container named "autoclave" and according to which the materials of the multilayer circuit board are covered with a film or a sheet and, after reducing the pressure inside the film or the sheet, the materials of the multilayer circuit board are heated and pressed by N.sub.2 gas or CO.sub.2 gas within the high pressure container.
In addition, Japanese Patent Unexamined Publication No. 3-128195 discloses a method in which heat and pressure are applied uniformly to the materials of the multilayer circuit board by the hot plates serving to heat the materials of the multilayer circuit board and by upper and lower bolsters serving to press the materials of the multilayer circuit board, a space formed between the upper and lower bolsters is enclosed by sealing means, the inside of the sealing means is kept at a vacuum (reduced pressure) until the bonding resin of the prepregs softens to fluidify and, after the bonding resin becomes into fluidified state, the pressure in the sealing means is increased to a high pressure (added with an air pressure) and kept at the high pressure during the hardening of the bonding resin, thereby making the pressure distribution uniform when the materials of the multilayer circuit board are bonded.
In the above-described step of bonding the materials of the multilayer circuit board, the best time for changing over from the vacuum atmosphere to the high pressure atmosphere is the time when the bonding resin of the prepregs softens to fluidify (when the viscosity of the bonding resin of the prepregs reaches the minimum viscosity). If the changing over time is advanced, it becomes liable to generate void. Further, even if no visible void is generated, the heat resistance and the stripping strength are inevitably deteriorated. On the other hand, if the changing over time is delayed, the bonding resin flows out in the peripheral portion of the materials of the multilayer circuit board to make the thickness of the formed multilayer circuit board smaller in the peripheral portion thereof than in the central portion thereof, thereby reducing the merit of the uniform-thickness forming that is the essential merit of the conventional air pressure added press described thirdly in the above. If the changing time is further delayed, solvent or gas in the hardened resin is induced to become the source of generation of void.
In addition, the time at which the bonding resin of the multilayer circuit board softens to fluidify in the bonding step is differed in accordance with an amount of materials of the multilayer circuit board inserted between the hot plates, the viscosity characteristic of the bonding resin, rate of temperature rise of the hot plates, heat capacities of jig plates and construction stores, and the like. Moreover, the temperature rise is differed between the portion close to the hot plate and the portion remote from the hot plate owing to the difference in heat transmission, resulting in a difference in the softening and fluidifying time described above. Particularly, in the case that the amount of materials of the multilayer circuit board inserted between the hot plates is large, a discrepancy in the above-described softening and fluidifying time becomes large. To cope with this, it is general to try to decrease the difference in temperature between the inserted materials of the multilayer circuit board by adding several sheets of cushion papers to the materials adjacent to the hot plate, adjusting the rate of temperature rise of the press body and the like means. As described above, estimation of the time at which the viscosity of the bonding resin is minimized is difficult and full of mistakes.
However, in the conventional bonding operation of the materials of the multilayer circuit board, although these circumstances have been considered to a certain extent, the working conditions have been decided according to the experience and sense of workers. In consequence, the amount of unevenness in quality of the formed multilayer circuit boards was large. Further, the unevenness in quality is brought about an increase of cost of the formed multilayer circuit board.
The unevenness in quality is brought about by non-uniform pressing by the upper and lower bolsters as well.
Namely, even if the contacting surfaces between the upper and lower hot plates and the materials of the multilayer circuit board facing each other are parallel with each other, there is caused a pressure distribution that the pressure is the largest in the central portion of the circuit board materials and becomes smaller as going toward the peripheral portion thereof attributable to a friction generated between the hot plates and circuit board materials. Such pressure distribution is generated at both contacting surfaces between the hot plates and the circuit board materials. The pressure is transmitted to heat insulating boards provided between the hot plates and the bolsters. The Young's modulus of each heat insulating board is very small as being less than about a hundredth of that of an iron-base material (hot plate material), so that the heat insulating board is easy to deform. For this reason, each heat insulating board is deformed in a shape that the thickness thereof is small in the central portion but large in the peripheral portion. Owing to such deformation of the heat insulating boards, the multilayer circuit board has a large thickness in the central portion but a small thickness in the peripheral portion, so that it is easy for void to remain inside the multilayer circuit board.
To solve the above problem, as disclosed in Japanese Patent Unexamined Publication No. 59-191600, it has been conventionally proposed to form a multilayer circuit board of a uniform thickness by providing between the bolster and the heat insulating board a thickness adjusting mechanism which is capable of partly changing the height so as to correct the deflection of the hot plate caused by the deformation of the heat insulating board.
However, by using the thickness adjusting mechanism, only the portion in which the thickness adjusting mechanism is provided was pressed strongly but the other portions were beyond adjustment, and accordingly, the pressure distribution could not be made uniform. As a result, there was formed a multilayer circuit board the thickness of which was corrected partly and hence which was hard to be referred to as multilayer circuit board of a uniform thickness.